Voice and audio band applications often include selectable paths for selecting different inputs and outputs, and generally include switches to select these inputs and outputs. In addition, if analog to digital conversion is required, a switched capacitor circuit is often used, including sampling switches to store the analog samples on the capacitors. In many applications, an MOS transistor is often used as a switch device. In a CMOS technology a transmission gate can be used to establish the switch.
FIG. 1 is a simplified view diagram illustrating a conventional switch device 100. As shown, switch circuit 100 has a PMOS transistor 110 with two terminals connected to terminals 101 and 102 of switch device 100, respectively. Typically, an MOS transistor is usually symmetrical in structure with respect to its source and drain terminals. Accordingly, either one of the terminals of the PMOS transistor 110 can function as a source or drain terminal, depending on the direction of current flow, or the relative magnitude of the voltages at terminals 101 and 102, respectively. PMOS transistor 110 also includes a gate terminal 121 that controls the flow of the current. Additionally, PMOS transistor 110 also has a body or bulk terminal 122.
Switch devices having only one switch transistor, such as switch device 100, can suffer from a number of limitations. For example, gate terminal 121 of PMOS switch 100 is often biased at ground and body terminal 122 at a high voltage supply. As the input signal drops and approaches the threshold voltage Vtp, switch 100 starts to turn off. Therefore, the useful magnitude of the input signal is limited and switch circuit 100 can not operate in the full supply voltage range. Additionally, with a fixed body bias, the performance of switch device 100 can be degraded because of raised effective threshold voltage due to body effect and leakage current caused by forward biased source to body junction at various operating conditions. Similarly, a switch device having a single NMOS switch transistor can also be susceptible to such limitations.
The operating range of input voltage in an analog switch can be improved with a CMOS switch device. FIG. 2 is a simplified schematic diagram illustrating a conventional CMOS switch device 200. As shown, switch device 200 has an NMOS transistor 210 and a PMOS transistor 220. Each of transistors 210 and 220 has two terminals connected to terminals 201 and 202 of CMOS switch device 200, respectively. As noted above, either one of the terminals of an MOS transistor can function as a source or drain terminal, depending on the direction of current flow, or the relative magnitude of the voltages at terminals 201 and 202, respectively. Additionally, NMOS transistor 210 has a gate terminal 211 and PMOS transistor 220 also has a gate terminal 221 that controls the flow of the current.
A CMOS switch is usually controlled by complementary control signals Vg and its complement signal Vg bar. When Vg is low, both transistors are off, and the switch is open. In FIG. 2, when Vg is high, both transistors are on, creating a low-resistance path. The body terminal of NMOS transistor 210 is usually biased at the lowest voltage, whereas the body terminal of PMOS transistor 220 is usually biased at the highest voltage. CMOS switch device 200 has a greater dynamic analog signal range in the On state as compared with switch device 100. However, as a bi-directional switch CMOS switch device 200 still faces the problems of body effect, including raised effective threshold voltage due to body effect and leakage current caused by forward biased source to body junction at various operating conditions.
Various techniques have been proposed to resolve the problems in analog switch circuits caused by body effects. As discussed in detail below, these conventional techniques are usually not satisfactory. For example, conventional techniques often require additional circuitry or place a high demand on certain circuit components, and therefore, can be more complex and expensive.
Accordingly, it is desirable to provide simple and cost-effective techniques for improving analog switch circuits.